Vacuum system cooling trap



T0 VACUUM CHAMBER Aug. 18, 1964 Filed July 25, 1962 K. W. ARNOLD ETALVACUUM SYSTEM COOLING TRAP 2 Sheets-Sheet 1 T0 DIFFUSION PUMP Aug. 18,1964 K. w. ARNOLD ETAL 3,144,756

VACUUM SYSTEM COOLING TRAP Filed July 23, 1962 2 Sheets-Sheet 2 ;E; 225-5 i -I Fig. 3

United States Patent 3,144,756 VACUUM SYSTEM COGLING TRAP Kenneth W.Arnold, Reading, and Richard B. Britten, Biiierica, Mass, assignors toion Physics Corporation, Burlington, Mass., a corporation of DelawareFiled .luly 23, 1962, Ser. No. 211,812 4 Claims. ('31. 62-469) Thisinvention relates to vacuum systems employing diffusion pumps and thelike, and more particularly to a cooling trap therefor adapted topresent a maximum condensation surface to vapors which diffuse onto itfrom the pump and from the vacuum chamber.

conventionally, a system to be exhausted, such as a high vacuum tank orparticle accelerator, is reduced to a rough vacuum by a rotarydisplacement mechanical pump which discharges into the atmosphere. Highvacuum is then obtained by means of a condensation or diffusion pump.This latter type of pump, in essence, comprises a container of a fluidsuch as mercury, heavy petroleum oil or chlorinated hydrocarbons. Thecontainer is heated sufficiently to vaporize the fluid which streams outof jets in the container walls entraining gas from the system beforebeing condensed by the cooled walls of the pump. Diffusion pumps,however, suffer from back-streaming of the operating fluid and are usedwith a trap cooled by solid carbon dioxide or liquid nitrogen tominimize this. It is toward the improvement of such traps that thisinvention is directed. Prior art devices of this type are characterizedby a metal container which is disposed in the vacuum system between thediffusion pump and the vacuum tank and filled with liquid nitrogen, air,or helium. The upper interior of such a trap is connected through aconduit to the atmosphere to allow disposal of the liquid as itevaporates. Since it is very desirable that such a trap present amaximum condensation surface to vapor diffusing onto it from the pump,the evaporation of the liquid contained within it limits itseifectiveness over any extended period of time. That is, as the liquidevaporates the level in the container drops, thus reducing the totalcontainer area in immediate contact with such liquid. Furthermore,conduction through the metal container is of little significance sincemetals required for vacuum purposes, such as stainless steel, have verypoor conductivity characteristics. The problem is still furthercomplicated by the nature of the liquid coolant, which being normally atvery low temperatures,

trap.

It is another object of this invention to provide a vacuum systemcooling trap of the type described adapted to present a constant maximumcondensation surface during the utilization time of each container ofliquid coolant.

It is another object of this invention to provide, in a cooling trap ofthe type described, a liquid coolant container that is of simplehydraulic design and that maintains its coolant contiguous to its entiresurface area.

It is another object of this invention to provide a cooling trap of thetype described that requires a minimum of maintenance.

It is another object of this invention to provide, in a vacuum systemcooling trap, a liquid coolant container having a separate compartmentadjacent to the walls and top thereof and means for maintaining amaximum amount of coolant therein.

It is another object of this invention to provide, in

1 3,144,756 Patented Aug. 18, 1964 Ice a vacuum system cooling trap, aliquid coolant container having a separate compartment adjacent thesides and top thereof in combination with a novel self-compensatingvalve means adapted to maintain coolant therein at a predeterminedlevel.

These together with other objects and features of this invention willbecome readily apparent from the following detailed description thereoftaken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a sectional view of one presently preferred embodiment ofthe invention;

FIGURE 2 is a detail illustrating a novel self-compensating pressurevalve adapted to use in such an embodimerit;

FIGURE 3 is a sectional view of an alternate embodiment of theinvention.

Referring now to FIGURE 1, there is illustrated thereby a cooling trapembodying principles of this invention. Vacuum system housing sections 4and 6 connect the subject cooling trap to the vacuum chamber (not shown)and to [the diffusion pump (not shown) respectively. Section 5, whichcomprises the housing of the trap is connected therebetween by vacuumseals 31. Container 7 is positioned within section 5 by any convenientmeans such as tubular members 10 and 11 or by metal struts, shouldgreater mechanical strength be required. Such struts should, however, befabricated of a metal that has poor heat conduction characteristics andthat is readily outgassed for vacuum use. Container 7 is preferably ofstainless steel or other metal that is impervoius to mercury vapor andthat is acceptable for vacuum use. The geometric configurations andrelationship of housing section 5 and container '7 should be such thatmercury (or other pump fluid) molecules arriving from the diffusion pump(indicated by arrows 15) must strike the cold surface of container 7 atleast once. A metal partition 8, preferably of copper, is provided toestablish a compartment 12 within the container that is substantiallycoextensive with the sides and top thereof. A plurality of apertures 9are disposed around the lower portion of metal partition as illustrated.The apex of inner compartment 14 is connected to the atmosphere throughconduit 10 and pressure valve 16. The apex of compartment 12 isconnected to the atmosphere through open conduit 11. Conduits lil and 11pass through the vacuum system housing by means of vacuum seals 33 and32 respectively.

Conduit 11 is made sufiiciently long (dimension h must be equal to orgreater than dimension h so that with pressure valve 16 set to maintainthe liquid level in compartment 12 at its maximum, no liquid will beforced to spill over the outer end of said conduit 11 regardless of theamount of liquid in the system.

In operation, the device is fieled with a liquid coolant 13 such asliquid nitrogen for instance. This is done conveniently by pouring theliquid nitrogen into conduit 11 until it appears in conduit 10 at valve16. Pressure valve 16 is then placed over the end of conddit 10. At thispoint the entire outer surface of the container is very cold and mercuryvapor striking it condenses and is thus prevented from passing to thehigh vacuum chamber. The liquid nitrogen 13, of course, due to the heatcreated by impinging mercury molecules slowly vaporizes and the vaporpasses to the atmosphere through conduit 11. In the conventional coldtrap, such evaporation of the liquid coolant results in a steadilydeclining liquid level and a consequently declining cold surface area.In the present invention, however, the vaporized fluid of compartment 14cannot escape into the atmosphere due to pressure valve 15, and thusbuilds up a pressure which forces liquid through apertures 9 to replaceliquid of compartment 12 lost by evaporation. The novel container ofthis invention in this way maintains a constant area ass maximum coldarea until substantially all of the liquid coolant has been utilized.Should the pressure in compartment 14 exceed that required to maintainthe coolant in compartment 12 at the proper level, pressure valve 16permits the escape of vapor through conduit in sufiicient quantity toprevent any overflow through conduit 11. A novel pressure valve for usein conjunction with the present invention is illustrated in FIGURE 2.Such a pressure valve is self-compensating and maintains a pressurewithin the inner chamber adapted to keep the coolant level in outerchamber 12 always at level c. Thus, in combination with the uniquecontainer structure, it represents another feature of the invention.Having particular reference to FIGURE 2, the valve comprises verticallydisposed conduit 19, compensating rod 21, rigid connecting member 20,and valve cap 18. Valve cap 18 is provided with heat conduction fins toprevent excessive amounts of frost from collecting thereon. It has beendetermined that the area A of valve cap 18 upon which the pressure ofinner compartment 14 acts must be equal to the cross sectional area A ofcompensating rod 21. Also, the length of compensating rod 21 shouldcoincide with the liquid level difference between full and emptyconditions of inner chamber 14 (/1 minus I1 or levels a and b). Withthese conditions fixed, it is only necessary to determine the combinedweight W of compensating rod 21, connecting member and valve cap 18.This is achieved simply in accordance with the relationship W=h P A inwhich P is the density of the coolant liquid. By way of example,assuming the conditions: [1 cm.; h :2.5 cm.; A=1 crrr and the coolantbeing liquid nitrogen of density P=0.8; the weight W would equal 25 10.8 or 20 grams. The length of the compensating rod would be 22.5 cm.and its volume would be 22.5 cm}. If 11 grams of the weight is in thecompensating rod and the rest in the accessories, the density of the rodmust be 11/22.5 or 0.49 gram/cc. It would be convenient to make thecompensating rod of aluminum plate 0.5 mm. thick, bent into the form ofan air-tight cylinder 22.5 cm. long with a 1 cm. cross sectional area.Its weight therefore would be 11.0 grams and the rest of the accessorieswould weigh 9.0 grams. In practice, to ensure stability of thecompensating rod it is necessary that the center of mass thereof isbelow the center of buoyancy. The valve, in operation acts in thefollowing manner. As the liquid in inner compartment 14 recedes from itsfull capacity (level b) to empty (level a) increasingly greater pressureis required therein to maintain the liquid in outer compartment 12 atlevel 0. Because of the particular weight and geometry of thecompensating rod and the buoyancy effects of the liquid, valve cap 18maintains just the right pressure within inner compartment 14 to supportthe liquid in outer compartment 12. As the liquid level in the innercompartment drops there is less buoyancy effect upon the compensatingrod and it effectively becomes heavier thus requiring a higher pressureto activate the valve.

Another embodiment of this invention is illustrated by FIGURE 3. In thisarrangement liquid is pumped from the inner compartment 14 to the outercompartment 26 by means of tube member 25 in combination with conicalmember 23 as illustrated. The partition 22 which separates inner andouter compartments in this embodiment does not have apertures in thebottom as does the embodiment of FIGURE 1. Conduit 21 provides anexhaust for evaporated liquid from both compartments and no pressure isrequired for operation. Conical member 23 has a plurality of openings 24around the bottom to permit the ready flow of liquid thereunder. Inoperation, a heat fiux generated by the impinging of mercury vapor(indicated by arrows 15) upon the bottom of the container causes bubblesto form within conical member 23. These bubbles congregate and move upthe sloping side of the conical member to for ma large bubble whichforces liquid up tube 25 and into the outer compartment.

It is to be understood that the above-described arrangements areillustrative of the applications of the principles of this invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the scope of the invention.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

1. In a vacuum system cold trap a container for liquid coolant having apartition therein, said partition being substantially parallel to thesides and top of said container and adapted to divide the interiorthereof into first and second compartments, said first compartment beingcontiguous to the sides and top of said container and having meansconnected to the apex thereof to permit emission of evaporated coolantformed therein, and said second compartment being contiguous to thebottom of said container and comprising a reservoir for said coolant,and means for pumping coolant from said second compartment to said firstcompartment.

2. A container for liquid coolant as defined in claim 1 wherein saidmeans for pumping coolant comprises, in combination, a conical member,said conical member having its base substantially co-extensive with andresiding on the bottom of said second compartment, and a capillary tube,said capillary tube being connected between the apex of said conicalmember and the apex of said first compartment and adapted to permit thepassage therethrough of said coolant.

3. In a vacuum system cold trap, a liquid coolant con-' tainer having aninner partition adapted to establish a first volume between itself andthe top and walls of said container and a second volume between itselfand the bottom of said container, said partition having a plurality ofapertures therethrough in the vicinity of the bottom of said container,a first vapor escape conduit connecting said first volume to theatmosphere, a second vapor escape conduit connecting said second volumeto the atmosphere, and a pressure valve, said pressure valve effecting aclosure of said second vapor escape conduit and having a pressurerelease response adapted to create a vapor pressure within said secondvolume whereby sufli cient liquid coolant is forced through saidpartition apertures to maintain said first volume in a full condition.

4. In combination with a vacuum system including a high vacuum diffusionpump and a chamber to be evacuated, a cold trap comprising, a housingmember, said housing member being an integral part of the vacuum systemand constituting a passage between said diffusion pump and said chamberto be evacuated, and a container for liquid coolant aflixed within saidhousing member, said container having an inner partition establishing arst compartment between itself and the top and sides of said containerand a second compartment between itself and the bottom of saidcontainer, said partition having a plurality of apertures proximate thebottom thereof, a first conduit attached to the top of said containerand effecting a communicating relationship between said firstcompartment and the atmosphere, a second conduit attached to the top ofsaid partition and effecting a communicating relationship between saidsecond compartment and the atmosphere, and a pressure valve, saidpressure valve effecting a closure of said second conduit.

References Cited in the file of this patent UNITED STATES PATENTS2,039,817 MacMillan May 5, 1936 2,052,410 Kucher Aug. 25, 1936 2,293,263Kornemann Aug. 18, 1942 2,317,814 Schuchmann Apr. 27, 1943 2,389,168Snyder Nov. 20, 1945 2,462,793 De Groat Mar. 29, 1949 2,853,859 ThompsonSept. 30, 1958 2,985,356 Beecher May 23, 1961 3,009,629 Garin Nov. 21,1961 3,057,340 Fritts Oct. 4, 1962

1. IN A VACUUM SYSTEM COLD TRAP A CONTAINER FOR LIQUID COOLANT HAVING APARTITION THEREIN, SAID PARTITION BEING SUBSTANTIALLY PARALLEL TO THESIDES AND TOP OF SAID CONTAINER AND ADAPTED TO DIVIDE THE INTERIORTHEREOF INTO FIRST AND SECOND COMPARTMENTS, SAID FIRST COMPARTMENT BEINGCONTIGUOUS TO THE SIDES AND TOP OF SAID CONTAINER AND HAVING MEANSCONNECTED TO THE APEX THEREOF TO PERMIT EMISSION OF EVAPORATED COOLANTFORMED THEREIN, AND SAID SECOND COMPARTMENT BEING CONTIGUOUS TO THEBOTTOM OF SAID CONTAINER AND COMPRISING A RESERVOIR FOR SAID COOLANT,AND MEANS FOR PUMPING COOLANT FROM SAID SECOND COMPARTMENT TO SAID FIRSTCOMPARTMENT.